Apparatus for clamping a work piece

ABSTRACT

The apparatus comprises a chuck member to be mounted within the operating area of the machine tool and a work piece carrier that can be put onto the chuck member and clamped thereto. First positioning members provided on the chuck member and second positioning members provided on the work piece carrier cooperate to define the position of the work piece carrier along three coordinate axes each running perpendicular to each other as well as with regard to the angular orientation. A clamping mechanism fixes the work piece carrier member to the chuck member in the afore mentioned defined position. In order to ensure an increased resistance against tilting momenta and torque occurring during the machining of a work piece, the first positioning members are constituted by conical centering pins and the second positioning members are constituted by recesses in the shape of a two-stage groove having two shoulders.

BACKGROUND OF THE INVENTION

The present invention refers to an apparatus for clamping a work piecein a well defined position within the operating area of a machine tool,comprising a chuck member adapted to be fixed within the operating areaof the machine tool, a work piece carrier adapted to be fixed to thechuck member and to be clamped thereto, and a clamping mechanism forclamping the workpiece carrier to the chuck member.

Such an apparatus, well known in the art, primarily serves for clampinga work piece to be machined in the operating area of a machine tool withvery high accuracy. It is particularly important that also therepetitive accuracy is ensured. In other words, the work piece receivedon the work piece carrier has to be clamped subsequently many times on achuck member adapted to the work piece carrier and shall thereby havealways a precisely defined position in X-direction, in Y-direction, inZ-direction and also regarding the angular orientation around theZ-axis, for example in circumstances if the work piece carrier bearingthe work piece to be machined has to be clamped in different machinetools that subsequently perform different machining operations and areeach equipped with an identical chuck member, or in measuring and teststations equipped with the same chuck member as well.

PRIOR ART

Such an apparatus is disclosed, for example, in the European Patent Nr.0,255,042. That apparatus comprises two pairs of positioning stripmembers towering above the surface of the chuck member, equipped withcontact surfaces for the orientation of the work piece carrier in X- andY-directions. Moreover, four pins are provided also towering above thesurface of the chuck member. These pins are responsible for defining theZ-position of the work piece carrier. The matching work piece carriercomprises a plane surface adapted to rest against the afore mentionedpins provided on the chuck member. Furthermore, the surface of the workpiece carrier is provided with two pairs of grooves in a layoutcorresponding to the one of the afore mentioned positioning stripmembers. The grooves have flexible lips that are intended to restagainst the contact surfaces of the afore mentioned strip members of thechuck member. Finally, the work piece carrier has a central aperture forreceiving a pulling bolt member by means of which the clamping forcerequired to maintain the defined position between work piece carrier andchuck member is transferred. Thereby, the chuck member comprises acentrally located ball lock mechanism cooperating with that pulling boltmember.

A disadvantage of such a known apparatus may be seen in the fact that itis relatively unstable, particularly if bigger and heavier,respectively, work pieces have to be machined. Thus, it cannot resisthigh tilting momenta or torque forces that can occur especially in thecase of cutting operations. An increase of the size of the apparatusthat would also increase the stability is not desired in many cases dueto limited space availability.

OBJECTS OF THE INVENTION

Thus, it is an object of the present invention to provide an apparatusfor clamping a work piece in a well defined position within theoperating area of a machine tool in which the work piece carrier clampedto the chuck member and, thereby, the work piece to be machined has asubstantially increased resistance against tilting momenta and torqueforces, simultaneously avoiding the danger that the work piece carrierand, thereby, the work piece is displaced relative to the chuck member,all this by maintaining the desired small size and a very high accuracyas far as the positioning of the work piece carrier relative to thechuck member is concerned, particularly also in the case of repetitiveclamping, as explained herein before.

SUMMARY OF THE INVENTION

In order to meet these and other objects, the invention provides anapparatus for clamping a work piece in a well defined position withinthe operating area of a machine tool, comprising a chuck member adaptedto be fixed within the operating area of the machine tool, a work piececarrier adapted to be fixed to the chuck member and to be clampedthereto, and a clamping mechanism for clamping the workpiece carrier tothe chuck member and adapted to be switched from a released condition toa clamping condition and vice versa.

The chuck member comprises first positioning members and the work piececarrier comprises second positioning members. The first and secondpositioning members cooperate such as to define the position of the workpiece carrier relative to the chuck member along three coordinate axeseach running perpendicular to each other, i.e. the X-axis, the Y-axisand the Z-axis, as well as with regard to the angular orientation aroundthe Z-axis.

The clamping mechanism thereby fixes the work piece carrier to the chuckmember once it is switched to the clamping condition to maintain thedefined position.

The first positioning members are constituted by conical centering pinsand the second positioning members are constituted by recesses in theshape of a two-stage groove. The groove has two shoulders whose edgesprotrude towards the interior of the groove. The mutual distance ofthese edges is somewhat less than the width of the conical centeringpins, measured between those locations that touch these edges once thework piece carrier is clamped to the chuck member.

In order to even further increase the desired high resistance againstunintentional positional changes caused by tilting and torque momentaoccurring under heavy load, it is provided in a preferred embodimentthat the centering pins are evenly distributed along the edge of the topsurface of the chuck member, whereby at least the peripherally oppositelateral surfaces of the centering pins are inclined with regard to theZ-axis of the apparatus. Moreover, the first positioning members furthercomprise elevated surface portions located on the top surface of thechuck member and extending up to the edge thereof.

An even still further increased resistance to withstand high torqueforces can be achieved if the recesses are machined into a shoulderprovided in an upper portion of the work piece carrier, whereby theshoulder protrudes towards the interior of the hollow cylindrical workpiece carrier, the location of the recesses corresponding to thelocation of the centering pins provided on the chuck member.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, an embodiment of the apparatus according to theinvention will be further described, with reference to the accompanyingdrawings, in which:

FIG. 1 shows a perspective view of a chuck member;

FIG. 2 shows a top view of the chuck member of FIG. 1;

FIG. 3 shows a perspective view of a work piece carrier member;

FIG. 4 shows a bottom view of the work piece carrier member of FIG. 3;

FIG. 5 shows a sectional view of the work piece carrier member of FIG. 4taken along the line V—V in FIG. 4; and

FIG. 6 shows a schematical partial sectional view of a centeringaperture and a centering pin in an enlarged scale.

FIG. 7 shows a sectional view of the chuck member of FIG. 1 taken alongthe line VII—VII in FIG. 2;

FIG. 8 shows a sectional view of the chuck member of FIG. 1 taken alongthe line VIII—VIII in FIG. 2;

FIG. 9 shows a sectional view of the chuck member of FIG. 1 taken alongthe line IX—IX in FIG. 2;

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The apparatus for clamping a work piece in a well defined positionwithin the operating area of a machine tool shown as an embodiment inthe drawings comprises, on the one hand, a chuck member, generallydesignated with reference numeral 1 and shown in FIGS. 1, 2 and 7-9,and, on the other hand, a work piece carrier member, generallydesignated with reference numeral 25 and shown in FIGS. 3-5. While thechuck member is essentially adapted to be fixed within the operatingarea of a machine tool (not shown in the drawings), the work piececarrier member 25 serves for receiving and holding a work piece to bemachined. It should be emphasized that the expression “work piece” shallbe understood in a general sense; particularly, a work piece could alsobe a tool to be machined, for example an electrode for an electroerosive machining device.

As can be seen particularly in FIGS. 7-9, the chuck member 1 essentiallycomprises three portions, i.e. a base portion or base element 2, anintermediate portion 3 and a head portion 4. In the embodiment shown inthe drawings, the base element 2 has the shape of a circular disc orplate member provided with a central aperture 5; the purpose of thataperture will be explained further herein after. The intermediateportion 3 is provided with a central aperture 6 having the same diameteras the aperture 5 and has the shape of a cylindrical or tube-likeelement. It is connected to the base element 2 by means of screws 7.Finally, the head portion 4 has a central aperture 8 as well, again withthe same diameter as the apertures 5 and 6. It has essentially pot-likeor hat-like shape with an inner diameter that is greater than the outerdiameter of the intermediate portion 3. The head portion, on the onehand, is connected to the base element 2 by means of screws 9 and, onthe other hand, is connected to the intermediate portion 3 by means ofscrews 10. Finally, the head portion 4 is provided with an outerperipheral shoulder 20. Inserted into the top surface of that shoulder20 is a sealing member 21.

By virtue of the afore mentioned design, an extremely stiff andflexurally rigid chuck housing is realized that moreover can be easilydisassembled, for example for maintenance reasons.

The upper surface 24 of the head portion 4 is provided with fourprotruding centering pins 22 that have essentially prismatic shape andthat are located evenly distributed along the edge region of the uppersurface 24 of the head portion 4. Two opposite centering pins 22 definethe extension of the X-axis, while the remaining two opposite centeringpins 22 define the extension of the Y-axis running perpendicularly tothe X-axis. The point of intersection of the X-axis and the Y-axisdefines the Z-axis (FIG. 7-9) running perpendicularly both to the X-axisand the Y-axis. As will be further explained herein after, at least thetwo peripherally opposite lateral surfaces of the centering pins 22 aresomewhat inclined with regard to the Z-axis.

Moreover, the top surface 24 of the head portion 4 is provided with fourplane surface portions 23 each of them extending, in the presentembodiment, radially between two adjacent centering pins 22. The surfaceportions 23 slightly tower above the top surface 24 of the head portion4. These surface portions 23 serve as Z-references, as will be furtherexplained herein after.

The design of the work piece carrier 25 is shown in FIGS. 3-5. As can beseen particularly in FIGS. 3 and 5, the one-piece work piece carrier 25comprises a peripheral portion 26 having essentially cylindrical ortube-like shape as well as an annulus-shaped top portion 27. The innerdiameter of the peripheral portion 26 essentially corresponds to theouter diameter of the head portion 4 of the chuck member 1, of coursewith a reasonable clearance such that the work piece carrier 25 can beput onto the chuck member 1.

The inner wall of the peripheral portion 26 of the work piece carrier 25is provided with a circular groove 28, whereby the two side walls of thecircular groove 28 extend conically away from each other as seen in adirection towards the interior of the peripheral portion 26. In otherwords, the circular groove 28 has an essentially V-shaped cross section.The purpose and function of this circular groove 28 will be furtherdiscussed herein after.

The upper portion 27 of the work piece carrier 25 is provided with anannulus-shaped shoulder 29 protruding towards the interior of theperipheral portion 26. Thereby, the mean diameter of this shoulder 29essentially corresponds to the diameter of the circle a long which thefour centering pins 22 provided on the head portion 4 of the chuckmember 1 are located. The front surface of that shoulder 29 thatprotrudes towards the interior of the peripheral portion 26 of the workpiece carrier 25 is smoothly plane ground and serves as a Z-reference ofthe work piece carrier 25. It is adapted to cooperate with the aforementioned plane surface portions 23 provided on the top surface 24 ofthe head portion 4 of the chuck member 1. Moreover, the aforementionedfront surface is provided with four recesses 30 evenly distributed alongthe periphery of that front surface. The exact design of these recesseswill be further explained herein after. Anyway, these four recesses 30are adapted to cooperate with the afore mentioned four centering pins 22provided on the top surface 24 of the head portion 4 of the chuck member1.

Even if four centering pins 22 and four recesses 30 cooperatingtherewith are shown and described in the present example, it isunderstood that it is also possible to provide three, six, eight or agreater number, being a mathematical sub-set of a Hirth gearing, ofcentering pins 22 and recesses 30 in order to ensure the positioning ofthe work piece carrier 25 with regard to the chuck member 1 in X- andY-directions as well as regarding the angular orientation.

FIG. 6 shows a schematical partial sectional view of a centering recess30 and a centering pin 22 in an enlarged scale. The recess 30 isdesigned as a two stage groove and comprises two shoulders 31 a and 31b. As already mentioned before, the centering pins 22 are slightlyconical by the fact that at least the two peripherally opposite lateralsurfaces of the centering pins 22 are somewhat inclined with regard tothe Z-axis. In practice, the angle of inclination can be in the regionof between 3° and 9°. The edges 32 a and 32 b of the shoulders 31 a and31 b protruding towards the interior of the groove 30 are authoritativefor the definition of the position of the work piece carrier 25 withregard to the chuck member 1 in X- and Y-direction as well as regardingthe angular orientation. Thus, the distance between these edges 32 a and32 b is somewhat less than the width of the conical centering pin 22engaging that recess 30, measured between the contact points or lines ofthe two opposite lateral surfaces of the pin 22 when the work piececarrier 25 is firmly clamped to the chuck member 1.

The mutual dimensions of the work piece carrier 25 and the chuck member1 are chosen such that the following consequence results:

If the work piece carrier 25 is loosely laid onto the chuck member 1,the centering pins 22 engage the corresponding centering recesses 30,whereby the lateral surfaces 22 a of the centering pins 22 get into linecontact with the edges 32 a and 32 b, respectively, of the shoulders 31a and 31 b, respectively. In this condition, the plane ground endsurface of the shoulder 29 of the work piece carrier 25, serving as theZ-reference of the carrier 25, does not yet abut against the planesurface portions 23 provided on the head portion 4 of the chuck member1, towering somewhat above its top surface and serving as theZ-reference of the chuck member 1. Rather, in this situation, there is agap between the end surface of the shoulder 29 and the surface of theportions 23 with a width of some {fraction (1/100)} of a millimeter. Onthe other hand, and this fact has to be emphasized, the position of thework piece carrier 25 with regard to the chuck member 1 in X- andY-directions as well as regarding the angular orientation is finallyset.

Once the work piece carrier 25 is loosely laid onto the chuck member 1and finally positioned with regard to the X- and Y-directions as well aswith regard to its angular orientation, the clamping mechanism, to bedescribed in the following, is operated such that the work piece carrier25 is pulled against the chuck member 1, in practice with a force in theregion of between 2000 and 3000 Newtons. Under the influence of thisclamping force acting in Z-direction, the shoulders 31 a and 31 b,respectively, are elastically deformed in the region of their edges 32 aand 32 b, respectively, particularly to such an extent until the endsurface of the shoulder 29 of the work piece carrier 25 abuts againstthe slightly elevated surface portions 23 of the head portion 4 of thechuck member 1. The result is that the work piece carrier 25 is exactlypositioned with regard to the chuck member 1 in Z-direction as well.

The clamping force required for clamping the work piece carrier 25against the chuck member 1 is created in the present embodiment asfollows:

By virtue of the already mentioned fact that the inner diameter of thehead portion 4 is larger than the outer diameter of the intermediateportion 3, a circular chamber 11 is created between these two portions 3and 4. The circular chamber 11 is sealed by means of suitable sealingelements indicated in the drawings (cf. FIGS. 7-9), but not referencedin detail. In the interior of this circular chamber 11, a circularpiston 12 is received in such a manner that it can be moved along thecentral axis Z of the chuck member 1. The circular piston 12 is providedwith a plurality of blind holes 13 that are evenly distributed along itscircumference and that are adapted each to receive a tensioning orclamping spring 14. The springs 14 rest against the bottom surface ofthe head portion 4 and bias the circular piston 12 to move downwards.Moreover, the circular piston 12 is provided with an outer peripheralgroove 15. The lower lateral wall of that groove 15 slopes outwardlydownwards. That groove 15 serves for partially receiving a plurality ofclamping ball members 18.

Below the circular piston 12, there is provided a hollow circularchamber 16 communicating with a connecting bore 17 provided in the baseelement 2. By means of this connecting bore 17, the circular chamber 16can be filled with pressurized air, acting on the circular piston 12 andmoves the latter one upwards against the pressure of the springs 14.

The head portion 4 is provided with a plurality of radially extendingbores 19 having a diameter that essentially corresponds to the diameterof the clamping ball members 18 and serving for receiving these clampingball members 18. The height level of these bores 19 and the height levelof the afore mentioned peripheral groove 15 are in such a relation thatthe clamping ball members 18 inserted into the bores 19 can enter theperipheral groove 15 if the circular piston 12, under the influence ofthe pressurized air in the circular chamber 16 and against the force ofthe springs 14, is moved in its topmost position. Thereby, the clampingballs 18 can penetrate the peripheral groove 15 to such an extent thatthe clamping balls no longer tower above the outer periphery of the headportion 4.

The axial position of the circular groove 28, i.e. the height level withreference to the Z-axis, is defined such that the clamping balls 18,schematically indicated in FIG. 5, can penetrate the groove 28 if thework piece carrier is loosely laid onto the chuck member 1. In thissituation, the clamping balls 18 are somewhat axially offset downwardlywith regard to the axially extending symmetry plane S—S of the circulargroove 28. In other words: If the work piece carrier 25 is loosely laidonto the chuck member 1, the afore mentioned symmetry plane S—S of thecircular groove 28 is located somewhat above the center of the balls 18inserted into the bores 19 of the intermediate portion 3 of the chuckmember 1.

The axial position of the circular groove 28 in the work piece carrier25 is located in such a way that the clamping balls 18 of the chuckmember 1 can penetrate the groove 28 if the work piece carrier 25 isloosely laid onto the chuck member 1, whereby the symmetry plane S—S ofthe circular groove 28 is located somewhat above the centers of theclamping balls 18 inserted into the radially extending bores 19 of theintermediate portion 3 of the chuck member 1. On the other hand, inorder to enable the work piece carrier 25 to be loosely laid onto thechuck member 1, the clamping balls 18 of the chuck member 1 have to bein their retracted position. This is accomplished (cf. FIGS. 7-9) byfeeding pressurized air into the circular chamber 16 below the circularpiston 12, with the result that the circular piston 12 is forced to anupward movement against the pressure of the springs 14. Thus, theperipheral groove 15 provided in the circular piston 12 is moved into aposition where it is aligned with the clamping balls 18 such that thelatter ones can penetrate the groove 15 and do not tower above thesurface of the intermediate portion 3 of the chuck member 1.

After the work piece carrier member 25 having been laid onto the chuckmember 1 and is positioned in X- and Y-directions as well as regardingits angular orientation, the overpressure in the circular chamber 1 isremoved with the result that the circular piston 12 is moved downwardsunder the influence of the force exerted by the springs 14. Thus, theclamping balls 18 are pressed into the circular groove 28 of the workpiece carrier 25. Due to the afore mentioned slightly asymmetric restposition between clamping balls 18 and circular groove 28 and due to theV-shaped design of the circular groove 28, as seen in a cross sectionalview, the work piece carrier 25 is pulled against the chuck member 1such that, as has already been mentioned, the end surface of theshoulder 29 of the work piece carrier 25 abuts against the slightlyelevated surface portions 23 of the head portion 4 of the chuck member1. The result is that the work piece carrier 25 is exactly positionedwith regard to the chuck member 1 in Z-direction as well and theclamping operation is finished.

From these explanations, it is evident that the clamping force actingparallel to the Z-direction is transferred by means of the clampingballs 18 to the work piece carrier 25 to clamp the latter one againstthe chuck member 1, particularly, in the present example, at twelvelocations evenly distributed along the periphery of the intermediateportion 3 of the chuck member 1. In other words, the entire clampingforce acting onto the work piece carrier 25 is summed up of twelveindividual clamping force components each transferred by one of thetwelve clamping balls 18. Thereby, the lines of activity of theseclamping force components are located on a cylindrical nappe MF (cf.FIGS. 2, 5 and 7-9) that runs, depending on the chosen dimensioning ofchuck member 1 and work piece carrier 25 as well as depending on theexact position of the centering pins 22, the recesses 30 and the endsurface of the shoulder 29, through these elements 22, 30 and 29, i.e.intersects these elements, or runs immediately next to these elements,preferably slightly outwards thereof. Thereby, it is ensured that theclamping force acts exactly at those locations where it is reallyrequired, with the consequence that the flexural stress present in thework piece carriers of the prior art is drastically reduced and entirelyavoided, respectively. Moreover, tilting momenta occurring during themachining of a work piece are much better absorbed than in the case of awork piece carrier of the prior art with a central clamping device.

It is understood that the circular piston 12 can be operated, instead bymeans of pressurized air, also hydraulically.

The final shaping of the recesses 30 of the work piece carrier 25preferably is performed by plastic deformation, for example by stamping.Thereby, a very high accuracy can be achieved, even in seriesproduction. The massive centering pins 22 having slightly sloped lateralsurfaces ensure, in cooperation with the recesses 30, that even veryhigh torques can be transmitted.

To the stability and the rigidity of the system chuck member/work piececarrier in the clamped condition contributes also the circumstance thatthe work piece carrier, including its centering elements in X-, Y, Z-and angular directions, is of a one piece construction. By virtue of theessentially pot- or hat-like shape of the work piece carrier 25, theessential parts and elements of the chuck member 1 are enclosed once thesystem is in operation, whereby particularly the centering elements 22,23 and 30 are well protected against contamination. Such protection iseven improved by the provision of the sealing member 21 (cf. FIGS. 7-9)located at the top side of the shoulder 20, against which the lower endsurface of the peripheral portion 26 of the work piece carrier 25 rests.

The provision of the central apertures 5, 6 and 8, respectively, in thebase element 2, in the intermediate portion 3 and in the head portion 4,respectively, in connection with the generally annular shaped design ofthe work piece carrier 25, enable also elongate work pieces to beclamped and machined. For example, the turbine blades of turbine rotorscan be received in the afore mentioned apertures in order to machine theso-called “fir tree” of the turbine blades.

If one can go without the afore mentioned central openness of theclamping system according to the present invention, it is possible toprovide an additional central clamping mechanism in order to furtherincrease the clamping force exerted to the work piece carrier, or toprovide a further, independent clamping system known in the prior art,for example to clamp smaller work pieces.

What is claimed is:
 1. An apparatus for clamping a work piece in a welldefined position within the operating area of a machine tool,comprising: a chuck means adapted to be fixed within the operating areaof the machine tool; a work piece carrier means adapted to be fixed tosaid chuck means and to be clamped thereto; clamping means for clampingsaid work piece carrier means to said chuck means and adapted to beswitched from a released condition to a clamping condition and viceversa; said chuck means comprising first positioning means and said workpiece carrier means comprising second positioning means, said first andsecond positioning means being adapted to cooperate such as to definethe position of said work piece carrier means relative to said chuckmeans along three coordinate axes each running perpendicular to eachother as well as with regard to the angular orientation; said clampingmeans fixing said work piece carrier means to said chuck means once itis switched to said clamping condition to maintain said definedposition; said first positioning means comprising conical centering pinmeans and said second positioning means comprising recess means in theshape of a two-stage groove means having two shoulder means, the edgesof said shoulder means protruding towards the interior of said groovemeans and being separated by a distance that is less than the width ofsaid conical centering pin means, measured between those locations thattouch said edges once said work piece carrier means is clamped to saidchuck means such that a clamping force provided by said clamping meanselastically deforms said edges of said shoulder means to enable asurface of said work piece carrier to abut against elevated surfaceportions of said chuck means.
 2. An apparatus according to claim 1 inwhich said centering pin means are evenly distributed along the edge ofthe top surface of said chuck means, whereby at least the peripherallyopposite lateral surfaces of said centering pin means are inclined withregard to the Z-axis of the apparatus.
 3. An apparatus according toclaim 2 in which the angle of inclination of said lateral surfaces ofsaid centering pin means amounts to between 3° and 9°.
 4. An apparatusaccording to claim 1 in which said elevated surface portion means arelocated on the top surface of said chuck means.
 5. An apparatusaccording to claim 1 in which said chuck means is of essentiallycylindrical shape and said work piece carrier means essentially has theshape of a hollow cylinder that is covered at one end by an essentiallyannulus-shaped plate member, such that said chuck means is enclosed bysaid work piece carrier means once the apparatus is in its clampedcondition.
 6. An apparatus according to claim 1 in which said recessmeans are machined into a shoulder means provided in an upper portion ofsaid work piece carrier means, said shoulder means protruding towardsthe interior of said hollow cylindrical work piece carrier means,whereby the location of said recess means corresponds to the location ofsaid centering pin means provided on said chuck means.
 7. An apparatusaccording to claim 1 in which said second positioning means furthercomprise plane machined surface portion means of said shoulder meansprovided on said work piece carrier means, said surface portion meansbeing adapted to rest against said elevated surface portion meansprovided on the top surface of said chuck means.
 8. An apparatusaccording to claim 1 in which both said chuck means and said work piececarrier means have central aperture means for receiving elongate workpieces.
 9. An apparatus according to claim 1 in which said clampingmeans comprises peripherally acting clamping device means with aplurality of clamping ball means evenly distributed along the peripheryof said chuck means, and said work piece carrier means comprising acircular groove means located at its inner cylindrical surface, saidclamping ball means being adapted to cooperate with said circular groovemeans.
 10. An apparatus according to claim 9 in which said chuck meanscomprises a circular chamber means and said clamping means comprises acircular piston means biased by spring means and being axially movablymounted in said circular chamber means, said clamping ball means restingagainst said circular piston means, whereby said circular piston meansis adapted to press said clamping ball means into said circular groovemeans provided in said work piece carrier means once said circularpiston means is axially moved.
 11. An apparatus according to claim 9 inwhich said clamping ball means are received in radially extending boresprovided in said chuck means and evenly distributed along its periphery.12. An apparatus according to claim 9 in which said clamping devicemeans comprises twelve clamping ball means.
 13. An apparatus accordingto claim 9 in which said circular groove means provided in said workpiece carrier means is of essentially V-shaped cross section.
 14. Anapparatus according to claim 9 in which the axial location of saidcircular groove means provided in said work piece carrier means isdefined such that said clamping ball means can penetrate said circulargroove means provided in said work piece carrier means when said workpiece carrier means is loosely laid onto said chuck means, whereby theplane of symmetry of said circular groove means provided in said workpiece carrier means is located slightly above the centers of saidclamping ball means inserted into said radially extending bores.